This invention relates to aqueous dispersions of rosin and to methods of preparing such aqueous dispersions. It also relates to the use of such aqueous dispersions in the manufacture of sized paper products and to paper products sized with the dispersion of rosin.
Millions of pounds of rosin are used annually for internal sizing of paper. Internal sizing, sometimes referred to as "beater sizing" or "engine sizing" is the addition of certain chemicals to paper slurry prior to sheet formation to produce paper that has resistance to water and water systems. In general, sizing imparts three main properties to paper, i.e., water resistance, temporary wet strength and feathering resistance. As an example, paper for drinking cups is sized so that hot coffee, water and other beverages do not soak through the paper. Writing paper is sized, otherwise ink would "feather" and the writing would be difficult to read. Paper bags and other wrapping papers, such as paper used to wrap meat, are given temporary wet strength as well as to resist wetting.
The two important chemicals used in internal sizing are rosin and alum. Typically gum rosin, wood rosin and tall oil rosin are used in papermaking, and the art has developed processes tailored to produce suitable sizes from all three of these rosins. Rosin size is made by saponifying rosin with soda ash or caustic soda. The amount of caustic may be varied to give different amounts of free (unsaponified) rosin. The less free rosin, the easier the size is to emulsify. Too little caustic gives a size with high free rosin and sizes with little free rosin are more completely neutralized. Those skilled in the art stabilize sizes with very high free rosin by adding casein or alpha-protein to the rosin emulsion. Since both the rosin size and the cellulose fibers are negatively charged, they repel each other and sizing would be impossible without another agent to attract both the size and the fibers. When alum is added, the positively charged aluminum ions attract these negative materials. The cellulose fibers then become covered with tiny, discreet particles of aluminum rosinate, which after drying, are water repellant.
Many types of rosins and saponified rosins suitable for internal sizing are known to those skilled in the art. As an example, U.S. Pat. No. 2,393,179 discloses a rosin size dispersion in which practically all of the rosin is present in free, i.e., unsaponified form, which is prepared by dispersing the free rosin with non-alkaline dispersing agents and stabilizing the dispersion so formed using a non-alkaline protective colloid. Typical non-alkaline dispersing agents include: sulfonated fat or oils, such as sulfonated castor oil, sulfonated stearin or neutral dispersing agents such as triethanol amine oleate, alkylaryl sulfonic acid salts, sulfonated higher fatty alcohols and the like. The protective colloid employed as stabilizing agents include neutral proteinaceous materials such as milk, casein, egg albumin and the like, although gum arabic, starch, water soluble methyl cellulose and the like may be employed.
U.S. Pat. No. 2,326,610 relates to oil in water emulsions wherein the dispersed phase comprises a substantial amount of free polymerized rosin. The polymerized rosin or blend may be emulsified by melting the polymerized rosin and bringing it into contact with a hot aqueous solution of a suitable alkali whereupon there is formed an alkali soap of polymerized rosin which acts as the emulsifying agent. Suitable emulsifying agents include any of the oil and water type, such as polymerized rosin, alkali soap, soaps of ordinary rosin, fatty acid alkali soap, sulfonated fatty glycerides, and the like.
U.S. Pat. No. 3,565,755 relates to rosins or fortified rosins prepared by preparing an essentially unstable, oil in water emulsion by mixing together water and the rosin or fortified rosins or salts thereof, and a solution of rosin base material dissolved in a water immiscible organic solvent. A stable oil in water emulsion is prepared by emulsification of the unstable emulsion and thereafter substantially all of the solvent is removed from the emulsion as by distillation to provide an aqueous suspension that is substantially homogeneous; and the saponified material, that is, the salt of rosin and/or salt of adduct reaction product serves as a dispersing agent in the aqueous suspension.
There are, however, certain attendant difficulties with either the manufacture or use of any of these sizing agents. Some of the prior art products will agglomerate in balls or lumps, causing the paper to stick to presses, dryers, felts, and the like, when the impregnated pulp is run out on a papermaking machine, or result in a mottled appearance of the sheet. Further, in many cases, coverage of the individual papermaking fibers may be poor, or in some cases, the precipitation of the rosin is incomplete and much of the material is lost in the white water. Although aqueous dispersion of a rosin, a fortified rosin, or a salt thereof that has been stabilized overcomes many of the above difficulties, such materials that are made by the use of a solvent have inherent difficulties such as the additional cost of manufacture to remove the solvent and the attendant fire hazards that are present during the preparation of the emulsion. These and other disadvantages of the prior art are overcome by the process of the present invention for preparing a stable aqueous dispersion of rosin, even containing substantially free rosin, that does not require the use of solvents during their preparation or the use of casein, alpha-protein or the like to stabilize the rosin.